1. Field of the Invention
The present invention relates to a semiconductor device having a circuit constituted by thin-film transistors (hereinafter referred to as TFTs). More particularly, the invention relates to an electronic device as represented by, for example, a liquid crystal display and to an electric appliance using such an electronic device as a display unit. In this specification, the semiconductor device refers to devices in general that work by utilizing the semiconductor characteristics. Therefore, electronic devices, semiconductor circuits and electric appliances are all semiconductor devices.
2. Description of the Related Art
Thin-film transistors (TFTs) can be formed on a transparent substrate and, hence, development has been positively forwarded for applying them to the active matrix-type liquid crystal display (hereinafter referred to as AM-LCD). The TFT using a crystalline semiconductor film (typically, a polysilicon film) exhibits a high mobility, and makes it possible to realize a highly precise image display by integrating functional circuits on the same board.
The AM-LCD basically comprises a pixel unit for displaying a picture, a gate drive circuit for driving TFTs of the pixels arranged in the pixel unit, and a source drive circuit (or a data drive circuit) for sending pixel signals to the TFTs, that are formed on the same substrate. In this specification, the gate drive circuit and the source drive circuit are all referred to as drive circuit units.
In recent years, there has been proposed a system-on-panel in which signal processing circuits such as signal-dividing circuit and xcex3-correction circuit, too, are formed on the same substrate in addition to the pixel unit and the drive circuit unit.
However, the pixel unit and the drive circuit unit require different circuit performances, and it is difficult to satisfy all circuit specifications by the TFTs of the same structure. That is, the structure of the TFT has not yet been established to simultaneously satisfy the TFTs (hereinafter referred to as pixel TFTs) that constitute the drive circuit unit such as shift register circuit which gives importance on the high-speed operation and that also constitute the pixel unit which gives importance on the high breakdown voltage characteristics.
The present applicant therefore has filed an application covering a constitution in which the gate insulating film has a different thickness depending upon the TFTs (hereinafter referred to as drive TFTs) forming the drive circuit unit and the pixel TFTs (Japanese Patent Laid-Open No. 10-056184). Concretely speaking, in the above application, the thickness of the gate insulating film of the drive TFTs is decreased to be smaller than the thickness of the gate insulating film of the pixel TFTs.
In recent years, each pixel in the pixel unit has been formed having an area as very small as about 18 xcexcmxc3x9718 xcexcm to realize a picture of as highly fine as XGA (eXtended Graphics Array, which is 1024xc3x97768 pixels) on a liquid crystal panel having a diagonal of 0.9 inch. It is expected that the size of the pixels will be further reduced in the future.
The greatest problem that stems from a reduction in the size of the pixels is a decrease in the numerical aperture in the transmission-type liquid crystal display. That is, the effective area for displaying the picture decreases, and the brightness decreases. If it is attempted to increase the numerical aperture, a countermeasure must be taken, such as decreasing the area occupied by the TFTs or decreasing the area occupied by the storage capacitors.
Thus, performance required for the TFTs and occupied areas are placed under very severe conditions accompanying a reduction in the size of the pixels and, besides, areas occupied by the storage capacitors are placed under severe conditions, too, making it very difficult to design the structure of the pixels.
The present invention was accomplished in view of the problems described above, and provides the structure of a pixel in which a highly reliable TFT is formed using a small area and the area occupied by the storage capacity is suppressed to a minimum degree. Thus, a bright and highly fine image picture is realized even by an electronic device of a pixel size which is as very small as a square of several tens of microns.
The invention further improves operation performance and reliability of the electronic device by suitably designing the structure of the TFTs arranged in the circuits depending on the functions of the circuits.
The invention further enhances operation performance and reliability of a semiconductor device (electric appliance) that uses the above electronic device (typically, a liquid crystal display, an electroluminescence display, an electrochromics display or a field emission display) as a display unit (display).
The constitution of the invention disclosed in this specification is concerned with a semiconductor device including a pixel unit and a drive circuit unit on the same substrate, wherein LDD regions of drive TFTs forming the drive circuit unit are so arranged as will be overlapped on the gate wirings of the drive TFTs to sandwich the gate-insulating films of the drive TFTs therebetween, LDD regions of pixel TFTs forming the pixel unit are so arranged as will not be overlapped on the gate wiring layers of the pixel TFTs so will not to sandwich the gate insulating films of the pixel TFTs therebetween, and the storage capacitors of the pixel unit are formed by a light-shielding film formed over the pixel TFTs, an oxide of the light-shielding film and pixel electrodes.
The constitution of the invention related to the method of manufacturing comprises:
a step for forming a channel-forming region, a source region, a drain region, and an LDD region sandwiched between the drain region and the channel-forming region in the active layers of NTFTs forming the drive circuit unit;
a step for forming a channel-forming region, a source region and a drain region in the active layers of PTFTs forming the drive circuit unit; and
a step for forming a channel-forming region, a source region, a drain region, and an LDD region sandwiched between the drain region and the channel-forming region in the active layers of pixel TFTs forming the pixel unit; wherein
the LDD regions of the NTFTs forming the drive circuit unit are so formed as will be overlapped on the gate wirings of the NTFTs forming the drive circuit unit to sandwich the gate-insulating films therebetween;
the LDD regions of the pixel TFTs are so formed as will not to be overlapped on the gate wirings of the pixel TFTs so will not to sandwich the gate insulating films therebetween; and
the storage capacitors in the pixel unit are formed by a light-shielding film formed over the pixel TFTs, an oxide of the light-shielding film and pixel electrodes.
More concretely, the invention is concerned with a method of manufacturing a semiconductor device which includes a pixel unit and a drive circuit unit on the same substrate, comprising:
a first step for forming an active layer on the substrate;
a second step for forming a gate-insulating film on the active layer;
a third step for forming an electrically conducting film on the gate-insulating film;
a fourth step for forming gate wirings of NTFTs that form the drive circuit unit by patterning the electrically conducting film;
a fifth step for forming n-regions in the active layers of NTFTs forming the drive circuit unit by adding an element belonging to the Group 15 of periodic table using the gate wirings of the NTFTs forming the drive circuit unit as a mask;
a sixth step for forming n-regions under the gate wirings of the NTFTs forming the drive circuit unit by diffusing the n-regions by heat treatment;
a seventh step for forming gate wirings of the pixel TFTs forming the pixel unit by patterning the electrically conducting film;
an eighth step for forming nxe2x88x92-regions in the active layers of the pixel TFTs by adding an element belong to the Group 15 of periodic table by using the gate wirings of the pixel TFTs as a mask;
a ninth step for forming n+-regions in the active layers of NTFTs forming the drive circuit unit and in the active layer of the pixel TFTs by adding an element belonging the Group 15 of periodic table;
a tenth step for forming gate wirings of the PTFTs forming the drive circuit unit by patterning the electrically conducting film;
an eleventh step for forming p+-regions in the active layers of PTFTs forming the drive circuit unit by adding an element belong to the Group 13 of periodic table by using the gate wirings of PTFTs forming the drive circuit unit as a mask;
a twelfth step for forming a interlayer-insulating film which is a resin film over the NTFTs and PTFTs forming the drive circuit unit and over the pixel TFTs forming the pixel unit;
a thirteenth step for forming a light-shielding film on the interlayer-insulating film;
a fourteenth step for forming an oxide of the light-shielding film on the surface of the light-shielding film; and
a fifteenth step for forming pixel electrodes in contact with the oxide of the light-shielding film and overlapped on the light-shielding film.